BETHESDA, MD. – U.S. officials leading the fight against Ebola say they know the ways the virus is spread and how to stop it.

They say that unless an air traveler from disease-ravaged West Africa has a fever of at least 101.5 or other symptoms, co-passengers are not at risk. "There is zero risk of transmission on the flight," Dr. Thomas Frieden, director of the federal Centers for Disease Control and Prevention, said after a Liberian man who flew through airports in Belgium and Washington was diagnosed with Ebola in Dallas last week. He died on Wednesday.

Other health officials voiced similar assurances, saying Ebola is spread only through physical contact or bodily fluids: "Ebola is not transmitted by the air," said Dr. Edward Goodman of Texas Health Presbyterian Hospital in Dallas, where the man was being treated.

Yet some scientists who have long studied Ebola say such assurances are premature — and they're concerned about what isn't known about the Ebola strain now on the loose. The present outbreak in West Africa has killed approximately 3,400 people.

Dr. C.J. Peters, who led the CDC's most far-reaching study of Ebola's transmission in humans, said he would not rule out the possibility that Ebola spreads through air. "We just don't have the data to exclude it," said Peters, who researches viral diseases at the University of Texas in Galveston.

Tom Skinner, a spokesman for the CDC in Atlanta, said that while the evidence "is really overwhelming" that people are most at risk when they touch sick people or have contact with their bodily fluids, "we can never say never" about the disease spreading through coughing or sneezing.

Moreover, some public health specialists said, there is no proof that an infected person without symptoms could not spread the virus. "It's really unclear," said Michael Osterholm, a public health scientist at the University of Minnesota who served on the U.S. government's National Science Advisory Board for Biosecurity. "None of us know."

CDC officials say asymptomatic patients cannot spread Ebola. Yet diagnosing symptoms can be subjective: Is a person mildly fatigued because of a lack of sleep or because of Ebola? Officials said last week that the patient hospitalized in Dallas, Thomas Eric Duncan, did not show signs of Ebola initially and failed to tell airport screeners he had contact with an Ebola-stricken woman. It is not known whether he knew she suffered from Ebola.

On Monday, President Barack Obama said his administration would develop additional screening protocols for airline passengers overseas and in the U.S. "I consider this a top national security priority," Obama said.

Skinner, at the CDC, said health officials were basing their responses on what was learned from fighting Ebola since 1976, when it was discovered in central Africa. He said ongoing studies would assess whether Ebola is mutating in ways that would require the government to change its policies. So far, he said, there has been no cause for concern.

SPAIN CASE: In Spain, three people were placed in quarantine Tuesday for possible exposure to Ebola. Spain's Health Ministry said Monday that a nursing assistant in Madrid tested positive. She is the first person in the current epidemic to have contracted Ebola outside Africa. The Associated Press reported that authorities plan to kill her dog, saying that scientific knowledge indicates a risk it could transmit the virus to humans. The New York Times said 50 others who might have had contact with her are being monitored. Health officials have said that the nurse became infected while treating a missionary who was in Sierra Leone.